The flat panel display devices possess many merits of thin frame, power saving, no radiation, etc. and have been widely used. Present flat panel display devices mainly comprise a LCD (Liquid Crystal Display) or an OLED (Organic Light Emitting Display).
An OLED possesses many outstanding properties of self-illumination, no requirement of backlight, high contrast, ultra-thin, wide view angle, fast response, applicability of flexible panel, wide range of working temperature, simpler structure and process, and, therefore is considered as a “dream display”. It has been favored by respective big display makers and has become the main selection of the third generation display element.
The OLED can be categorized into two major types according to their driving types, which are Passive matrix OLED (PMOLED) and Active matrix OLED (AMOLED), i.e. the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The power consumption of the PMOLED is high, and thus, it hinders the application in large scale display devices. Therefore, the PMOLED is generally applied for the small scale display devices. The lighting efficiency of the AMOLED is high, and therefore, it is generally utilized for the large scale display devices of high resolution.
FIG. 1 shows a circuit diagram of an AMOLED pixel circuit according to prior art. In the display area of the AMOLED display device, the pixels are arranged in an array comprising multiple rows, multiple columns. Each pixel generally utilizes a pixel circuit comprising two thin film transistors and one capacitor for performing driving, i.e. the 2T1C driving. The gate of the first thin film transistor T1 is electrically connected to the scan line Scan, and the source is electrically connected to the signal line Data, and the drain is electrically coupled to the gate of the second thin film transistor T2 and one end of a capacitor C; the source of the second thin film transistor T2 is electrically connected to the power source line VDD, and the drain is electrically connected to the anode of the organic light emitting diode D; the cathode of the organic light emitting diode D is electrically connected to the common ground electrode VSS; one end of the capacitor C is electrically connected to the drain of first thin film transistor T1, and the other end is electrically connected to the source of second thin film transistor T2. As displaying, the scan line Scan controls the first thin film transistor T1 to be on, and the signal voltage of the signal line Data enters the gate of the second thin film transistor T2 and the capacitor C via the first thin film transistor T1, and then the first thin film transistor T1 is off. With the function of capacitor C, the gate voltage of the second thin film transistor T2 still can keep the signal voltage to allow the second thin film transistor T2 to be in an on state, and the drive current corresponded with the power source line VDD and the signal voltage enters the organic light emitting diode D via the second thin film transistor T2 to drive the organic light emitting diode D to illuminate.
The aforesaid AMOLED display device is current driving. However, the lifetime of the organic light emitting diode D is not stable, and the properties of the organic light emitting diode D is changing along with the time and temperature, which can make the current flowing through the organic light emitting diode D change. The brightness can be changed thereby, and the change conditions of the respective pixels are different. Thus, the issue of uneven display happens.